1. Field of the Invention
The present invention relates to a method of detecting a frame edge, and more particularly to efficiently and speedily detecting an edge of a photographed frame (i.e., a boundary with a film base) at the time of printing an image from an original film, such as a negative film.
2. Statement of the Related Art
Hitherto, automatic photographic color printing apparatuses have been known in which printing is effected in such a manner that the density of all the finished prints and the color balance become identical regardless of the light and shade of a negative (i.e., underexposure, adequate exposure, and overexposure) by measuring the large area transmittance density (LATD) of the overall images of a color negative film (i.e., an original film) to compensate for the density and by effecting slope control. This automatic photographic printing apparatus is arranged by sequentially disposing an optical system provided with a light source, a light-adjusting filter, a mirror box, a negative carrier, a lens, and a black shutter. After the original film is placed on the negative carrier, the light source is lit, and the black shutter is then opened to allow an image of the original film to be formed on photographic paper via the lens, thereby effecting printing. The printed photographic paper is developed in a development process, and prints are automatically finished. In this automatic photographic printing apparatus, the light generated from the light source and transmitted through the original image is decomposed into the primaries of red light (R), green light (G), and blue light (B). An LATD is measured separately for each of R, G, and B to determine an amount of printing light on the basis of the principle of Evans. At the same time, slope control is carried out to compensate for reciprocity law failure, and the density of prints and color balance are thereby controlled.
In addition, in automatic photographic printing apparatuses, it is necessary to accurately position the frames of the original film on the printing apparatus in order to properly print frame images of the original film on photographic paper. To effect this positioning automatically, a notch is conventionally provided in a side end portion of each frame of the original film by using a notcher in a preceding process, and the notch is detected by a photosensor or the like to effect positioning. However, at the time of providing the notch, it is necessary to make the notch correspond to the frame accurately, so that there has been a drawback in that much labor is required. Furthermore, there is another method whereby positioning is effected by feeding the original film constantly by a fixed amount, i.e., a fixed distance. With this method, however, accuracy is poor since the misalignment at the time of film feeding for printing is accumulated, so that there is a drawback in that this method is not suitable for automatic processing.
To solve these problems, the present applicant has already proposed a method of automatically positioning the frame of a film at a printing position by obtaining high-resolution image information by determining data between adjacent picture elements by interpolation using an exposure-controlling two-dimensional image sensor having a relatively coarse image-element density, whereby a frame edge is detected accurately, and the frame of the film is automatically positioned at a printing position. In this method, an output of a picture element array is detected by pitches that are relatively smaller than the picture-element pitches of the two-dimensional image sensor, and the frame edge is detected by a statistical technique using a frequency distribution of interpolated variables. Accordingly to this method, if a leading frame closest to a film edge is positioned in advance at the time of setting a negative film for printing on a negative carrier, since the lengths of the frames and frame intervals are substantially fixed thereafter, the negative film frames can be automatically positioned without any problems in practical use, with the exception of the positioning work for the leading frame, by feeding the negative film by a predetermined amount and detecting the edge of the next frame within a predetermined range of detection, thereby effecting positioning. In the case of an elongated negative (strip negative), the leading frame which is subject to printing is generally located at a position removed from a leading edge portion of the negative film, and, if the edge of the leading frame is detected using the aforementioned method, the edge of the leading frame is precision-detected with pitches that are smaller than the picture element pitches. Consequently, it is necessary to fetch data continuously from the leading edge portion of the negative film with pitches that are smaller than the picture element pitches and to effect arithmetic processing so as to detect the position of the frame edge. Consequently, the range of precisely detecting the position of the leading frame is wide, so that a large amount of time for fetching data and processing time are required. Hence, there is a drawback in that the film cannot be conveyed at high speed, and the frame edge cannot be detected efficiently. In the case of a short (piece) negative as well, there are cases where the position of the leading frame is located in the same way as the elongated negative (a piece negative which is cut including the leading portion of the elongated negative) and cases where the position of the leading frame is adjacent to a film edge. In both cases, with the method of detecting the frame edge using a statistical method which uses the leading frame position as a reference, there is a drawback in that it is difficult to automatically discriminate these negatives. In addition, since the leading end portion of the negative film must be positioned manually to avoid these drawbacks, the efficiency becomes poor. Hence, in any case, there is the problem of causing a hindrance to automation.